An Ethernet card behaves like a ca - 80 mA current source (or a 80 mA
current sink if you like). Since the cable is terminated with 50 Ohm
resistors at both ends, this yields a voltage of (50 / 2) * -.080 = - 2
Volts.

The signal is fed to the twisted pair via a low pass filter, transformer and
common mode coil. The differential voltage is ca 2 ... 2.5 Vpp.

When there is no data the card sends a 100 ns pulse every 16 ms. After two
successive pulses the link LED goes on. No signal for 150 ms means the link is
down. Fast ethernet uses a more elaborate system.

Power over Ethernet

Since ethernet uses transformers, it is possible to transport power over
ethernet, in the same way ISDN does.
To this end 48 V DC is applied to the centre taps of the RX and TX transformers;